Since aluminum alloys are light and have high thermal conductivity, aluminum alloys are used for automobile heat exchangers such as radiators, condenser, evaporators, heaters or intercoolers. An automobile heat exchanger is mainly produced by a brazing method, and brazing is generally conducted using a brazing filler metal of an Al—Si based alloy at a high temperature around 600° C.
Various methods are employed as the brazing method. For example, a brazing method using a fluoride flux, which is a non-corrosive flux, in N2 gas is generally employed.
A three-layer tube material which includes an Al—Mn based alloy, typified by JIS3003 alloy, or the like as the core material has been generally used as a tube material in a heat exchanger in which a coolant circulates inside the tube, as in an automobile radiator or heater. Such a three-layer tube material is obtained for example by cladding a sacrificial anode material such as an Al—Zn alloy on the inner surface of a core material of JIS3003 alloy and cladding a brazing filler metal such as an Al—Si based alloy on the outer surface.
In a method for producing a tube used for a radiator or a heater, the end faces of a three-layer tube material are butted and continuously welded while fabricating the material into a tube, and after cutting and removing beads at the welded joints, the resulting tube is fabricated flat, thereby obtaining a flat tube.
Here, as the demand for lighter automobiles has been increasing recently, ways to reduce the weights of automobile heat exchangers and to reduce the thicknesses of respective parts constituting a heat exchanger have been investigated. In order to reduce the thicknesses of the parts, a material which has superior post-brazing strength and corrosion resistance to those of the conventional materials is needed.
Means for improving the strength include a method for improving the strength by adding Si and Cu in high amounts to the core material and a method for improving the strength by adding Mg to the sacrificial anode material. However, problems of quality, such as weld cracking, are more likely to arise during welding, due to the thickness reduction and the improvement of the strength, and a tube material with excellent weldability is desired.
For example, PTL 1 proposes an aluminum alloy brazing sheet strip in which the distribution of Mg2Si compounds formed around the core material/sacrificial anode material interface is defined to restrict local melting during welding. In addition, an aluminum alloy clad material in which the structure of the core material and the tensile strength of the clad material are defined is proposed in PTL 2 for example.
The weldability can be improved by the means to some extent. However, when the concentrations of the elements such as Mg and Cu, which are added to the core material and the sacrificial anode material to make the material thin and to obtain a high-strength material, at the grain boundaries are high, problems arise because the melting point of the grain boundaries decreases and the grain boundaries melt earlier during welding. When welding is conducted with many Al—Mg—Cu based intermetallic compound existing at the interface between the core material and the sacrificial material, there are also problems because the intermetallic compound melts earlier and the sacrificial material peels off. Therefore, it cannot be considered that the weldability is secured enough with the conventional techniques, and further improvement of the weldability of a thin/high-strength material is desired.
As described above, it has been difficult with the conventional techniques to provide a material which is thin but has excellent weldability and which has improved post-brazing strength.